Biochemical Synthesis of Steroid Hormones PDF

Biochemical Synthesis of Steroid Hormones  Learning Objectives Physiology Describe the biochemical pathway of synthesis of the glucocorticoid and mineralocorticoid steroid hormones from their common precursor cholesterol Differentiate between the key enzymes involved in...

Biochemical Synthesis of Steroid Hormones  Learning Objectives Physiology Describe the biochemical pathway of synthesis of the glucocorticoid and mineralocorticoid steroid hormones from their common precursor cholesterol Differentiate between the key enzymes involved in each of these pathways Describe the cellular mode of action of these steroid hormones Describe the effects of deficiencies of these steroid hormones Physiology Describe how these deficiencies result in congenital adrenal hyperplasia (CAH) 2  Clinic relevance; Congenital Adrenal Hyperplasia (CAH) Inherited autosomal recessive disorders Mutations in the genes encoding enzymes in mineralocorticoids, glucocorticoids or sex steroids synthesis Enlarged adrenal glands (hyperplastic adrenomegaly Results in reduced levels of cortisol or aldosterone with overproduction of androgen 3  Clinical presentation of classic CAH In infants: In children and adults: ▪Ambiguous genitalia in ▪ Early onset of puberty females ▪ Rapid growth during ▪Enlarged genitalia in males childhood ▪Poor weight gain ▪ Premature completion of ▪Adrenal crisis (dehydration, growth (short stature) vomiting, hypoglycaemia and circulatory collapse) ▪ Irregular menstrual cycles ▪ Infertility in both sexes 4  Classification of steroid hormones Glucocorticoids Mineralocorticoids 5  Classification of steroid hormones Glucocorticoids; cortisol - regulates metabolism, mood, blood pressure, immunity, pain sensation Mineralocorticoids; aldosterone – electrolyte and fluid balance, blood volume Androgens; testosterone – male reproductive development Estrogens; female secondary sexual characteristics, regulation of the menstrual cycle Progestogens; regulation ovulation, maintains pregnancy 6  Synthesis occurs in the adrenal cortex and gonads Zona Mineralocorticoids glomerulosa [aldosterone] Zona Cortex Glucocorticoids [cortisol] fasciculata Zona (Sex steroids [androgens]) reticularis Catecholamines [adrenaline Medulla & noradrenaline] 7  Synthesis occurs in the adrenal cortex and gonads Adrenal glands synthesise corticosteroids (mineralo- and glucocorticoids) and androgens Ovaries (depending on the stage of the ovarian cycle) secrete estrogens and progesterone Testis produce large amounts of androgens Biochemical pathways involved are strikingly similar in all tissues Series of tissue-specific enzymes located in the mitochondria and ER Steroid hormones are lipid soluble (freely permeable to membranes) so are not stored 8 Regulation of adrenal gland hormones Stressors e.g. Physical/physiological stress Corticotropin-releasing hormone Adrenocorticotropic hormone 9  Steroid hormones are derived from cholesterol Cholesterol (C27) Pregnenolone (C21) Progesterone (C21) Glucocorticoids (C21) Androgens (C19) Mineralocorticoids (C21) Estrogens (C18) 10 11  BASICS PRIMER In hydroxylation, a hydrogen atom in the substrate molecule is replaced by a hydroxyl group (-OH). This changes the chemical structure and properties of the molecule. Hydroxylation reactions in biological systems are typically catalyzed by enzymes. Many of these enzymes are from the cytochrome P450 family Eg. enzyme 21α-hydroxylase catalyzes the addition of a hydroxyl group (-OH) to the 21st carbon of progesterone. In this process, a hydrogen atom at the 21st position is replaced by a hydroxyl group. Oxidation: refers to a process where a molecule, atom, or ion loses electrons. In biochemistry, it often involves the addition of an oxygen atom or the removal of hydrogen atoms. Eg. 18-Hydroxycorticosterone is an intermediate in the synthesis of aldosterone. It contains a hydroxyl (-OH) group at the 18th carbon position. Oxidation Reaction>Action of 18-Hydroxydehydrogenase: This enzyme catalyzes the oxidation of the 18-hydroxyl group to an aldehyde group. This involves the removal of hydrogen atoms (a common type of oxidation in organic chemistry) and results in the conversion of the -OH group to an =O group (aldehyde). 12 Steroid hormones are derived from cholesterol Synthesized in the liver Lipid with a unique structure; four linked hydrocarbon hydrocarbon tail linked hydroxyl group Amphipathic molecule (hydrophilic and hydrophobic parts) 13  Steroid hormone synthesis summary Shortening of the hydrocarbon tail of cholesterol Hydroxylation of steroid nucleus Enzymes: Cytochrome P450 (CYP) mixed function oxidases Requires NADPH and oxygen Modification occurs in endoplasmic reticulum (ER) and mitochondria of adrenal gland, ovary and testes 14  First step in steroid synthesis StAR: steroidogenic acute regulatory protein controls the uptake of cholesterol into the mitochondria Cholesterol is converted to pregnenolone by the cholesterol side-chain cleavage Catalyzed by the enzyme, cytochrome P450 side-chain cleavage (also called desmolase, or CYP11A1) This is a rate limiting step; NADPH and oxygen are required, occurs in adrenal, ovary and testis 15  Progesterone is synthesised from pregnenolone Pregnenolone Enzymes 3-hydroxyl group (OH) is oxidised to 3-keto group 3-OH dehydrogenase 5,4 isomerase 5 double bond is isomerised to a 4 double bond Progesterone 16 3 beta=3b-OH dehydrogenase 17  Aldosterone is synthesised from progesterone Hydroxylation at C-21 on Progesterone progesterone by 21- Corticosterone hydroxylase to yield 11- 21 -hydroxylase deoxycorticosterone Hydroxylation at C-11 11  11-deoxycorticosterone hydroxylase to yield corticosterone 11 -hydroxylase Hydroxylation at C-18 on corticosterone by 18- Corticosterone hydroxylase and oxidation 18 hydroxylase Aldosterone of C-18 by 18-hydroxy- Aldosterone dehydrogenase to an 18 hydroxy- aldehyde to yield synthase dehydrogenase aldosterone Aldosterone Enzymes: 21 -hydroxylase, 11 -hydroxylase, aldosterone synthase 18 19  Cortisol is synthesised from progesterone Progesterone 17 -hydroxylase 17-Hydroxyprogesterone Hydroxylation order; C-17 21 -hydroxylase C-21 11 Deoxycortisol C-11 11 -hydroxylase Cortisol Enzymes:17 -hydroxylase, 21 -hydroxylase, 11 -hydroxylase 20  Summary of key enzymes Enyzme Reaction Location ER 3-OH-dehydrogenase Pregnenolone to progesterone 5,4-isomerase Pregnenolone to progesterone ER 17--hydroxylase Progesterone to 17-hydroxyprogesterone ER (CYP 17) a. 17-hydroxyprogesterone to 11- 21--hydroxylase deoxycortisol ER (CYP21A1) b. Progesterone to deoxycorticosterone 11--hydroxylase a. 11-deoxycortisol to cortisol Mitochondria (CYP11B1) b. Deoxycorticosterone to corticosterone 18-hydroxylase Corticosterone to aldosterone Mitochondria Aldosterone 18-hydroxy- Synthase Corticosterone to aldosterone Mitochondria dehydrogenase Know the shorthand and long-form of the enzyme names 21  Hormone EXTRACELLULAR Cellular Mode of (estrogen) FLUID Action of Steroid Estrogen Hormones receptor Plasma membrane Diffuse across the cell membrane Hormone-receptor complex NUCLEUS Bind intracellular receptor CYTOPLASM within the cytosol or the nucleus. DNA Protein Bind to segment of DNA mRNA trigging transcription of a target gene to mRNA 22  Hormone Response Elements These are regions of DNA – contain a consensus sequence Typically close (upstream/5’) to transcription initiation site HRE’s associate not only with activated receptor but also with nuclear receptor proteins (TF) – these provide further layer of complexity/control 23  Steroid hormone functions Steroid hormones play important primary functions in; Carbohydrate regulation (glucocorticoids) Mineral and fluid balance (mineralocortiocoids) Reproductive functions (gonadal steroids) Steroids also play role in inflammation, stress responses, bone metabolism, cardiovascular function, behaviour and mood 24 Metabolic functions of glucocorticoids Inhibit tissue building while stimulating the breakdown of stored nutrients to maintain adequate fuel supplies 25  Functions of Cortisol many functions in the human body, such as mediating the stress response, regulating metabolism, the inflammatory response, and immune function 26  Actions of Mineralocorticoids Secreted in response to increase ECF and high K+ concentration. Stimulates transcription of the sodium-potassium ATPase Increased numbers of sodium pumps in the basolateral membranes of tubular epithelial cells Facilitates uptake of sodium (and with it, water) from the tubular lumen Reabsorbs sodium in the kidney at the expense of potassium and H+ 27  Disorders of adrenal gland insufficiencies; Congenital Adrenal Hyperplasia (CAH) Autosomal recessive disorders of cortisol biosynthesis Failure to secretion cortisol – loss of negative feedback to HPA axis Depending on the specific enzyme step involved there may be distinct symptoms and lab findings Classification; 3β-dehydrogenase deficiency 17-a-hydroxylase 21-Hydroxylase deficiency 11β-Hydroxylase deficiency 28  CAH: Deficiency in 3-OH dehydrogenase No glucocorticoids, No mineralocorticoids, No active androgens X X X 29  CAH: Deficiency in 3-OH dehydrogenase No glucocorticoids, No mineralocorticoids, No active androgens Deficiency in 3β-OH dehydrogenase Virtually no glucocorticoids, mineralocorticoids active androgens or estrogens Salt excretion in urine Affected genetic males (46, XY) have ambiguous genitalia Autosomal recessive with incidence 1:10,000 30 CAH: Deficiency in 17--hydroxylase (CYP17) Low androgens and cortisol; Increase in aldosterone X X 31  CAH: Deficiency in 17--hydroxylase (CYP17) Low androgens and cortisol; Increase in aldosterone Deficiency in 17α-hydroxylase Virtually no sex hormones or cortisol are produced Increased production of mineralocorticoids causes sodium and fluid retention and therefore hypertension Genetic males (46, XY) have female-like genitalia 32  CAH: Deficiency in 21 -hydroxylase Most common: no cortisol, no aldosterone, increase in androgen X X 33  CAH: Deficiency in 21 -hydroxylase Most common: no cortisol, no aldosterone, increase in androgen Deficiency in 21α-hydroxylase Partial and virtually complete deficiencies are known Classic Form- Deficiency in production of mineralocorticoids and glucocorticoids. Salt-wasting form is the most severe, with virtually absent mineralocorticoids and glucocorticoids Non-Classic Form: mineralocorticoids might be normal or only slightly reduced, while glucocorticoids can be deficient to varying degrees. Overproduction of androgens leads to masculinization of external genitalia in females and early virilization in males 34  CAH: Deficiency in 11--hydroxylase (CYP11B1) Low cortisol and aldosterone; Overproduction of deoxycorticosterone (which has mineralocorticoid activity); Overproduction of androgens X X 35  CAH: Deficiency in 11--hydroxylase (CYP11B1) Low cortisol and aldosterone; Overproduction of deoxycorticosterone; Overproduction of androgens Deficiency in 11-β-hydroxylase (CYP11B1) Decrease in serum cortisol, aldosterone and corticosterone. Increased production of deoxy-coticosterone causes fluid retention due to its ability to supress the renin/angiotensin system; hypertension and hypokalemia Overproduction of androgens leads to masculinization of external genitalia in females and early virilization in males (e.g.- early development of pubic hair) 36  Clinical Effects of CAH Cortisol deficiency leads to ▪ Poor response to stress ▪ Hypoglycaemia ▪ Hyperpigmentation Aldosterone deficiency leads to ▪ Hyperkalemia ▪ Hyponatremia ▪ Disturbs cardiac rhythm ▪ Hypotension Combined deficiency can lead to vascular collapse, shock and death  Clinical Effects of CAH No negative feedback to hypothalamus ▪ Overproduction of ACTH ▪ Adrenal hyperplasia Excess androgen production ▪ Masculinisation of external genitalia in females No active androgens ▪ Female-like genitalia  Treatment for CAH Daily glucocorticoid and mineralocorticoid replacement Restore volume, electrolyte and glucose imbalances Reference material Lipincott’s Illustrated Reviews 5th Ed p237-244 Principles of Medical Biochemistry, Meisenberg and Simmons 2nd Ed p287 – 292 Article: Congenital adrenal hyperplasia. Speiser PW, White PC.. N Engl J Med. 2003 Aug 21;349(8):776-88. Review. PubMed PMID: 12930931